\begin{spreadtab}{{tabular}{r*6c}} % or \spreadtab{{tabular}{r*6c}}
@Ages & 10 & 11 & 12 & 13 & 14 & 15\\
@Number & 5 & 8 & 20 & 55 & 9 & 3\\\hline
@Average&\multicolumn{6}{l}{:={sumprod(b1:g1;b2:g2)/sum(b2:g2)}}
\end{spreadtab} % or \endspreadtab
